Customer awareness and sensitivity to noise and vibration levels have been raised through increasing television advertisement, in which the vehicle noise and vibration performance is used as the main market differentiation. This awareness has caused the transportation industry to regard noise and vibration as important criteria for improving market shares. One industry that tends to be in the forefront of the technology to reduce the levels of noise and vibration is the automobile industry. Hence, it is of practical interest to reduce the vibrations induced structural responses.
The automotive vehicle engine is the main source of mechanical vibrations of automobiles. The engine is vulnerable to the dynamic action caused by engine disturbance force in various speed ranges. The vibrations of the automotive vehicle engines may cause structural failure, malfunction of other parts, or discomfort to passengers because of high level noise and vibrations. The mounts of the engines act as the transmission paths of the vibrations transmitted from the excitation sources to the body of the vehicle and passengers. Therefore, proper design and control of these mounts are essential to the attenuation of the vibration of platform structures.
To improve vibration resistant capacities of engine mounting systems, vibration control techniques may be used. For instance, some passive and semi-active dissipation devices may be installed at mounts to enhance vibration energy absorbing capacity.
In the proposed study, a radically different concept is presented whereby periodic mounts are considered because these mounts exhibit unique dynamic characteristics that make them act as mechanical filters for wave propagation. As a result, waves can propagate along the periodic mounts only within specific frequency bands called the "Pass Bands" and wave propagation is completely blocked within other frequency bands called the "Stop Bands".
The experimental arrangements, including the design of mounting systems with plain and periodic mounts will be studied first. The dynamic characteristics of such systems will be obtained experimentally in both cases. The tests will be then carried out to study the performance characteristics of periodic mounts with geometrical and/or material periodicity. The effectiveness of the periodicity on the vibration levels of mounting systems will be demonstrated theoretically and experimentally. Finally, the experimental results will be compared with the theoretical predictions.
Active periodic structures exhibit unique dynamic characteristics that make them act as tunable mechanical filters for wave propagation. As a result, waves can propagate along the periodic structures only within specific frequency bands called the “Pass Bands” and wave propagation is completely blocked within other frequency bands called the “Stop Bands”. The spectral width of these bands can be tuned according to the nature of the external excitation.
In this paper, the emphasis is placed on developing a new class of these periodic structures called Active Periodic Struts (APS) which can be used to support gearbox systems on the airframes of helicopters. When designed properly, the APS can stop the propagation of vibration from the gearbox to the airframe within critical frequency bands and consequently minimizing the effects of transmission of undesirable vibration and sound radiation to the helicopter cabin. The theory governing the operation of this class of Active Periodic Struts (APS) is presented and their tunable filtering characteristics are demonstrated experimentally as function of their design parameters.
The presented concept of the APS can be easily employed in many applications to control the wave propagation and the force transmission both in the spectral and spatial domains in an attempt to stop/confine the propagation of undesirable disturbances.